starkernel: item 4.3.1 -- framebuffer orientation test, found and fixed a real color-swap bug

Adds fb_draw_orientation_test() (framebuffer.c/.h): fills the four raster
corners RED/GREEN/BLUE/YELLOW via fb_fill_rect. Wired into kernel_main.c
calling fb_init() directly -- console_fb_init()/vt100_init() removed from
the boot path, since vt100.c/console.c are superseded by the Console
drawing-fabric redesign (FABRIC.md ss27) and should not be exercised even
incidentally.

The diagnostic caught a real, pre-existing bug on its first run: framebuffer.c's
pack_pixel() had its FB_PIXEL_RGBX32/FB_PIXEL_BGRX32 branches swapped relative
to UEFI GOP's own byte-order naming convention, producing a clean R<->B channel
swap (G unaffected). Spatial placement was already correct -- no flip/rotation.
Fixed by swapping pack_pixel's two return bodies to match framebuffer.h's
already-correct doc comments; kernel_main.c's GOP-format switch needed no change.

Also item 4.3.2 -- QEMU screenshot capability. scripts/qemu_screenshot.sh
already existed (monitor socket + socat + HMP screendump), just unwired and
unused this session. Redirected its PNG output to a new top-level fb/
directory (tracked in git, not logs/, not a gitignored temp dir) and added a
python3+PIL fallback for PPM->PNG conversion since imagemagick isn't
installed here. Left as a standalone script for now, not wired into a
Makefile target.

FABRIC.md items 4.3.1 and 4.3.2 marked done with acceptance evidence.

scripts/qemu_screenshot.sh

@@ -9,8 +9,9 @@
 # Makefile target which already built the ISO during the serial acceptance run).
 #
 # Outputs:
-#   <BUILD_DIR>/screenshot.ppm              — raw PPM from QEMU screendump
-#   <LOG_DIR>/qemu-screenshot-YYYYMMDD-HHMMSS.{log,png}  — serial log + PNG
+#   <BUILD_DIR>/screenshot.ppm                 — raw PPM from QEMU screendump
+#   <LOG_DIR>/qemu-screenshot-YYYYMMDD-HHMMSS.log  — serial log
+#   fb/qemu-screenshot-YYYYMMDD-HHMMSS.png         — PNG (tracked in git, not build/)
 #
 # Requires: qemu-system-x86_64, socat, xorriso, mtools, ovmf, imagemagick
 
@@ -61,14 +62,15 @@ fi
 # --------------------------------------------------------------------------
 # Setup paths
 # --------------------------------------------------------------------------
-mkdir -p "${LOG_DIR}" "${BUILD_DIR}"
+FB_DIR="fb"
+mkdir -p "${LOG_DIR}" "${BUILD_DIR}" "${FB_DIR}"
 
 TS=$(date +%Y%m%d-%H%M%S)
 SERIAL_LOG="${LOG_DIR}/qemu-screenshot-${TS}.log"
 VARS_COPY="${BUILD_DIR}/OVMF_VARS_screenshot.fd"
 MONITOR_SOCK="${BUILD_DIR}/qemu-monitor-${TS}.sock"
 PPM_OUT="${BUILD_DIR}/screenshot.ppm"
-PNG_LOG="${LOG_DIR}/qemu-screenshot-${TS}.png"   # committed artifact
+PNG_LOG="${FB_DIR}/qemu-screenshot-${TS}.png"   # committed artifact
 QEMU_PID_FILE="${BUILD_DIR}/qemu-screenshot.pid"
 
 cp "${OVMF_VARS_RO}" "${VARS_COPY}"
@@ -144,9 +146,14 @@ if [ -f "${PPM_OUT}" ]; then
         convert "${PPM_OUT}" "${PNG_LOG}"
         rm -f "${PPM_OUT}"
         echo "  PNG        : ${PNG_LOG}"
+    elif command -v python3 >/dev/null 2>&1 && python3 -c "import PIL" >/dev/null 2>&1; then
+        python3 -c "from PIL import Image; Image.open('${PPM_OUT}').save('${PNG_LOG}')"
+        rm -f "${PPM_OUT}"
+        echo "  PNG        : ${PNG_LOG} (via python3/PIL)"
     else
-        echo "  WARN: imagemagick 'convert' not found — PPM only (install imagemagick)"
-        PNG_LOG="${PPM_OUT}"
+        echo "  WARN: no PPM->PNG converter found (imagemagick 'convert' or python3+PIL) — PPM only"
+        PNG_LOG="${FB_DIR}/$(basename "${PNG_LOG}" .png).ppm"
+        mv "${PPM_OUT}" "${PNG_LOG}"
     fi
     echo ""
     echo "=== Screendump complete ==="